The present invention is directed to semiconductor devices and, more particularly, to nitride-based semiconductor devices, such as Schottky diodes, and to processes for making the same.
Nitride-based semiconductors, such as gallium nitride and gallium nitride-based semiconductors, are widely regarded as desirable wide bandgap compound semiconductors. These materials have been adopted in optoelectronic devices, such as light-emitting diodes (LEDs), laser diodes and photodiodes, and have also been employed in non-optical electronic devices, such as field effect transistors (FETs) and field emitters. In optoelectronic devices, the wide bandgap of the material allows for emission or absorption of light in the visible-to-ultraviolet range. In electronic devices, gallium nitride and its related materials provide high electron mobility and allow for operation at very high signal frequencies.
The properties of nitride-based semiconductors also make such materials desirable for use in Schottky diodes. Schottky diodes are desired for applications where energy losses while switching from forward bias to reverse bias and back can significantly impact the efficiency of a system and where high current conduction is desired under forward bias and little or no conduction is desired under reverse bias, such as when used as an output rectifier in a switching power supply. The Schottky diodes have lower turn-on voltages because of the lower barrier height of the rectifying metal-to-semiconductor junction and have faster switching speeds because they are primarily majority carrier devices. Nitride-based semiconductors are thus highly desirable as Schottky diodes because of their high electron mobility which reduces the device on-resistance when the Schottky diode is forward biased and because of their ability to withstand high breakdown fields when reverse biased. Additionally, gallium nitride and gallium nitride-based semiconductors have the further advantage that the barrier height at the metal-to-semiconductor junction, and thus the forward voltage drop, varies depending upon the type of metal used in the junction.
The lowered metal-to-semiconductor barrier height, however, can increase the reverse leakage current when the metal-to-semiconductor junction is reverse biased. Therefore, a lower doped semiconductor layer is desired for the metal-to-semiconductor junction. The lower doped layer, however, results in a higher on-resistance when the device is forward biased. It is thus further desirable to incorporate a more highly doped layer that serves as the major part of the conduction path and to minimize the thickness of the lower doped layer, thereby reducing the resistance when the device is forward biased. A tradeoff therefore exists when attempting to reduce the forward resistance of a device as well as reduce the reverse breakdown voltage. When the Schottky diode is optimized for higher reverse-bias breakdown voltages, such as by increasing the resistivity and thickness of the lower doped layer, the on-resistance increases. By contrast, when the device is optimized for low on-resistance, such as by providing a more highly doped and thicker low resistance layer, the breakdown voltage decreases.
To optimize the Schottky diode for both high reverse-bias breakdown voltage as well as for low forward bias on-resistance, a thin, very low doped layer is desired to serve as the metal-to-semiconductor contact. Such low doping levels, however, are very difficult to attain in a repeatable manner that is uniform across the layer.
It is therefore desirable to provide a Schottky diode that has a very low doped layer that can be formed repeatably with uniform doping.